1. Field of the Invention
The present invention relates to a method for controlling the air/fuel ratio of an internal combustion engine with a fuel cut operation.
2. Description of Background Information
In order to accelerate the purification of the exhaust gas and to improve the fuel economy of an internal combustion engine, a feedback type air/fuel ratio control system is generally used. Oxygen concentration in the exhaust gas is detected and an air/fuel ratio of the mixture supplied to the engine is controlled to a target air/fuel ratio by a feedback control operation in accordance with a result of the detection of the oxygen concentration.
As an oxygen concentration sensor for use in such an air/fuel ratio control system, there is a type which generates an output signal whose level is proportional to the oxygen concentration in a gas whose oxygen concentration is to be measured. The detail of which is disclosed in Japanese Patent Application laid open No. 58-153155. With this type of oxygen concentration sensor generating an output signal proportional to the oxygen concentration, it is possible to control the air/fuel ratio very accurately.
On the other hand, if the engine rotation is maintained with the throttle valve closed, for example, when the engine is decelerating, the vacuum in the intake manifold will rise quickly due to the closure of the throttle valve. High vacuum may in turn lead to an increase of harmful components (especially, carbon monoxide CO) in the exhaust gas because the air/fuel ratio is very much enriched under such a condition. In order to prevent the increase of noxious components, a fuel cut operation may be used in some cases. In the case of a carburetor type fuel metering system, the fuel cut operation is performed by a low-speed circuit of the carburetor that stops the fuel supply to the engine. On the other hand, if the fuel metering system is of the type using a fuel injector, then the drive of the fuel injector is stopped for the fuel cut operation. In the case of the feedback type air/fuel ratio control system, the operation of the system is such that the feedback operation is stopped during the fuel cut operation, and the system resumes the feedback control operation upon completion of the fuel cut operation. During the fuel cut operation, the fuel that adheres to the inner walls of the intake manifold is sucked into the cylinders of the engine as a result of the increase of the vacuum in the intake manifold caused by the closure of the throttle valve. Also, the temperature in the combustion chamber drops during the fuel cut operation. Because of these reasons, the output signal level of the oxygen concentration sensor represents the concentration of an unburnt oxygen component immediately after a completion of the fuel cut operation. For example, as shown in FIG. 1A, the output signal level of the oxygen concentration sensor gradually decreases after a time t.sub.2 of the completion of the fuel cut operation. Therefore, if the feedback air/fuel ratio control which is based on the output signal of the oxygen concentration sensor is started immediately after the completion of the fuel cut operation, the air/fuel ratio of the mixture will be controlled to the then rich side as shown in FIG. 1B. This is because the air/fuel ratio of the mixture is detected under this condition to be leaner than a target air/fuel ratio in accordance with the output signal of the oxygen concentration sensor. The supply of such a rich mixture will result in the generation of unburnt components especially carbon monoxide CO and hydrocarbons HC.